Pressurized-water nuclear reactors comprise a core consisting of prism-shaped assemblies arranged side by side in vertical position. The assemblies comprise a framework consisting of longitudinal guide tubes and spacer grids and closed by connectors in which the fuel rods are arranged.
One of the connectors of the assemblies, called the lower connector, comes to rest on the lower core plate which is pierced with holes in the region of each of the assemblies, to allow the cooling water of the reactor to pass through the core vertically and from the bottom upwards.
This connector comprises supporting feet which come to rest on the lower core plate, and an element transverse relative to the direction of the fuel rods of the bundle fixed to the supporting feet. This transverse element consists of an adapter plate, in which the lower ends of the guide tubes of the assembly are fastened and through which extend orifices allowing the passage and ensuring the distribution of reactor cooling water which circulates in the vertical direction and in contact with the outer surface of the rods of the assembly, after passing through the lower connector.
The adapter plate of the lower connector of a fuel assembly of a water-cooled reactor thus contributes to the rigidity of the framework of the assembly, ensures the absorption of forces exerted on the lower connector and particularly the absorption of the weight of the framework and of the bundle transmitted by the guide tubes and must, furthermore, allow the passage and distribution of the cooling water of the assembly.
The adapter plate is mainly subjected to bending and undergoes high stresses, especially in the vicinity of its upper face and lower face. This plate, produced in one piece, must have a large thickness ensuring that the lower connector has high rigidity.
The adapter plate must also ensure that the fuel rods of the assembly are retained, in the event that some of these rods are no longer held sufficiently effectively by the holding elements of the spacer grids and tend to slide in the axial direction of the bundle under the effect of their own weight.
Insofar as the water passage holes extending through the adapter plate are generally of a diameter larger than the diameter of the fuel rods, it is necessary to provide a network of holes in positions offset relative to the transverse positions of the fuel rods, so that each of the fuel rods is vertically in line with a solid part of the adapter plate.
Even when all or some of the holes passing through the adapter plate have a diameter smaller, even markedly smaller, than that of the fuel rods, it is desirable to avoid placing these holes of small diameter vertically in line with the fuel rods which, in the event of an accidental fall, risk blocking one or more water passages and therefore reducing or locally eliminating the cooling of the fuel rods by water circulation.
Moreover, in view of the thickness of the adapter plate, it is necessary to provide water passage holes which are such that the head loss of the cooling water through the lower connector is as small as possible.
It is extremely difficult to design an adapter plate which can satisfy the various mechanical and hydraulic requirements mentioned above.
The machining of this adapter plate is an extremely difficult operation requiring high accuracy of execution.
Furthermore, debris particles may be present in the primary circuit of the reactor and are liable to be carried along by the circulating pressurized water. If they are of a small size (for example, smaller than 10 mm), they can pass through the adapter plate of the lower connector, the water passage holes of which have a diameter generally larger than 10 mm. Such debris may become jammed between the fuel rods and the elements holding the rods in the region of the first grid, i.e., the spacer grid holding the rods according to a regular network and being the lowest in the assembly. This debris subjected to the axial and transverse hydraulic stresses which are high in this zone, can wear the sheathing of the fuel rod. This risks a loss of sealing of this sheathing and an increase in the rate of activity of the primary circuit of the reactor.
To prevent this debris from being carried along inside the assembly, it has been proposed to carry out filtration of the cooling fluid in the region of the adapter plate of the lower connector by providing holes which pass through this adapter plate, and the diameter of which is sufficiently small to stop the debris liable to be jammed in the first grid of the assembly.
However, to ensure that the flow of cooling fluid passes through the adapter plate with only moderate head loss, it is necessary to provide a very dense network of passage holes, thus further complicating the machining of the adapter plate.
The lower connectors of the fuel assemblies can be produced in one piece by the machining of a molded part or, on the contrary, from a plurality of molded and machined parts which are joined together by welding. In this case, it can be especially advantageous to carry out the welding by electron beam.
The connectors of the assembly consist, for example, of a reticular structure limited externally by a frame, the cross-section of which corresponds substantially to the cross-section of the assembly, and having walls connected to one another and to the frame, delimiting large-size cells introducing only a negligible head loss into the circulation of the cooling water of the reactor. A plate of small thickness which can be die-stamped is attached and secured removably to one of the faces of the reticular structure. To regulate the flow of the cooling water through the connector, the plate is pierced with orifices, the shape, dimensions and arrangement of which make it possible to set the head loss and distribution of the water passing through the connector.
Such connectors are mainly used as upper connectors of the assemblies and are unsuitable for use as lower connectors in which they must ensure both the retention of the rods and the distribution and adjustment of the flow of cooling water entering the assembly. Moreover, these connectors do not ensure that debris transported by the reactor cooling water is stopped and retained.
These processes for producing lower connectors of fuel assemblies from attached parts therefore make it impossible to provide a completely satisfactory solution to the problems regarding the mechanical stability and hydraulic behavior of the adapter plates and the possibility of stopping and retaining the debris transported by the cooling water.